As we know, electrocatalytic water splitting through hydrogen evolution reaction (HER) is a promising and sustainable pathway for H₂ production [1-4]. Up to now, the high-performance catalysts reported could only achieve high HER activities in acidic or alkaline conditions; these catalysts are based on Pt and other noble metals, which are rare and precious [5, 6]. However, considering the demands on electrolysis equipment and the need for environmental protection, it is better to find efficient electrocatalysts for the HER in neutral media, which suffer from a large ohmic loss and low electron density of the neutral electrolyte [7]. Therefore, it is necessary to develop low-cost and widely applicable electrocatalysts for industrialization of hydrogen production in the near future.

To date, bimetal sulfide hybrids with diverse structures have exhibited attractive HER activities because of the reliability and plenitude of the active redox sites provided by both the metal sulfide components [8, 9]. Moreover, the interface between the two components brings about synergistic effects, which, owing to enrichment of the active sites and reconfiguration of the electronic structures, further enhance the HER activity, compared with those of their single-component counterparts [10-12]. For example, Zhang et al. [13] engineered the interface of MoS₂/Ni₃S₂ heterostructures with excellent water-splitting electrocatalytic properties in alkaline solutions. It has been confirmed that the lattice interfaces of NiS₂/MoS₂ heterojunctions are propitious to the dissociation of H₂O as part of all-pH HER [14]. Therefore, engineering of the interfacial structure between NiS₂ and MoS₂ is the key to further enhancing the HER activity in a wide pH electrolyte, particularly in alkaline and neutral conditions.

Here, we synthesized three-dimensional (3D) self-supported NiS₂/MoS₂ heterostructures on nickel foams by a facile method, namely, millisecond-laser-direct-writing; these structures can be directly used as an electrode for electrocatalyzing water splitting to produce H₂, and exhibited outstanding HER activities in both alkaline and neutral electrolytes. In this work, we provide a new strategy for interface control. Thanks to the high instantaneous temperature and rapid cooling in this method, the heterostructures easily constitute highly dispersed minor-sized NiS₂ nanoparticles and MoS₂ nanosheets that exhibit large specific surface areas and abundant heterointerfaces between NiS₂ and MoS₂, which ensure a larger amount of catalytically active sites compared to those obtained in other works. Additionally, the 3D porous micro/nanostructure is propitious to electrolyte permeation as well as H₂ diffusion, which further guarantees a distinguished electrocatalytic performance.

The synthetic method of catalysts, namely millisecond-laser-direct-writing [15], is illustrated in Scheme S1. The 3D NiS₂/MoS₂ heterostructures were formed by the ablation of the millisecond laser and the rapid quenching of plasma comprising elements from the nickel foam and (NH₄)₂MoS₄ solution. As shown in Fig. 1a, the products were grown in-situ on the nickel foam at locations that were laser treated. The X-ray diffraction (XRD) pattern has been provided in Fig. S1, and it indicates the existence of two metal sulfides, instead of doping. We can see nanospheres piled on the nickel foam surface (Fig. 1b), and the size of the nanospheres is about 100 nm, with the surface appearing rough or wrinkled (Fig. 1c). It is obvious that MoS₂ nanosheets with a thickness of about 5 nm interweaved in NiS₂ nanoparticles of sizes below 30 nm, and the two mixed irregularly in the nanospheres (Fig. 1d). As a result, a massive heterointerface developed in the nanospheres between NiS₂ and MoS₂ (Fig. 1e and S2). Lattice fringes with lattice distances of approximately 0.25 and 0.61 nm well corresponded to the (210) lattice plane of NiS₂ and the (002) lattice plane of MoS₂ (Fig. 1f), respectively. The fast Fourier transform patterns of MoS₂ and NiS₂ reveal their high crystallinities (inset of Fig. 1f), which match well with the XRD results. Energy dispersive X-ray spectroscopy (EDS) reveals the elemental composition of the sample (Fig. S2d). Moreover, scanning transmission electron microscopy results and the corresponding EDS mapping images show that nickel, molybdenum, and sulfur are well distributed in the NiS₂/MoS₂ heterostructures (Fig. 1g–j), which in turn indicates that NiS₂ and MoS₂ are well dispersed in the heterostructures.

XPS measurements were used to examine the chemical states of the elements in the NiS₂/MoS₂ heterostructures (Fig. S3). The Mo 3d spectrum of the NiS₂/MoS₂ heterostructures shows two significant peaks located at 229.5 and 232.7 eV, which are ascribed to Mo 3d_5/2 and Mo 3d_3/2, respectively, which indicate the dominance of Mo⁴⁺ in NiS₂/MoS₂ [16]. However, in the spectrum of pure MoS₂ nanosheets, these peaks appear at 229.2 and 232.5 eV (Fig. 1h). In the Ni 2p spectrum of the NiS₂/MoS₂ heterostructures, the Ni 2p_3/2 and 2p_1/2 peaks are observed at 855.4 and 872.9 eV, along with two satellite peaks, that represent Ni²⁺ [17]. Obviously, compared with the peaks of pure NiS₂, these peaks display a positive shift of nearly 0.4 eV (Fig. 1i). The positive shift in the Mo 3d and Ni 2p spectra of the NiS₂/MoS₂ heterostructures suggests the existence of strong electronic interactions between NiS₂ and MoS₂, which indicates the establishment of coupling interfaces [13], more precisely named as a heterointerface. Moreover, the XPS pattern of physically mixed NiS₂ and MoS₂ provides further support to the above conclusion (Fig. S4) because no peak shifts are observed.

To evaluate the HER catalytic activity of the NiS₂/MoS₂ heterostructures in alkaline media, we established a three-electrode system in 1 mol L^-1 KOH electrolyte. As shown in Fig. 2a, the overpotential of the NiS₂/MoS₂ heterostructures is 98 mV, with 10 mA cm^-2 constant cathodic current density (η₁₀) On the other hand, nanostructured NiS₂ and MoS₂ both show lower HER electrocatalytic activities, with η₁₀ of 221 and 320 mV, respectively, which reveal the necessity of the heterointerface. Besides, the curve of the nickel foam (η₁₀ = 253 mV) indicates that although the substrate plays an important role in gas diffusion, it is not the main reason for the improvement in the HER performance. Fig. 2b reveals the Tafel slope of the NiS₂/MoS₂ heterostructures as 88 mV dec^-1, which suggests that the HER follows the Heyrovsky mechanism. Further, this Tafel slope is notably smaller than those of NiS₂, MoS₂, and nickel foam, which were calculated as 178, 154, and 227 mV dec^-1, respectively. The outstanding HER activity reported above indicated that these heterostructures outperformed many transition metal sulfides in basic solutions (Table S1). Cyclic voltammograms were recorded at different scan rates in the range of non-Faradaic potentials (Fig. S6), and the capacitive currents were plotted against the scan rates, the slopes of which corresponded to the electrochemical double-layer capacitance (Cdl; Fig. 2c). In line with the low Tafel slope, the NiS₂/MoS₂ heterostructures exhibit the highest electrochemical active surface area (ECSA), based on the Cdl, which suggests the existence of more active sites in the NiS₂/MoS₂ heterostructures and smaller η₁₀ (Fig. 2d).

The EIS results of the NiS₂/MoS₂ heterostructures are shown in Fig. 2e. Obviously, compared with those of NiS₂, MoS₂, and nickel foam, the Nyquist plot of the NiS₂/MoS₂ heterostructures suggests a smaller charge transfer resistance (Rct), which is advantageous in improving the HER activity. As shown in Fig. 2f, a long-term test based on galvanostatic measurement at about 10 mA cm^-2 in 1 mol L^-1 KOH solution was carried out for 10 h to evaluate the stability of the NiS₂/MoS₂ heterostructures. Only 12% reduction from the original current density is observed for the NiS₂/MoS₂ heterostructures, and the polarization curves reveal no obvious changes after the long-term test (inset of Fig. 2f); on the other hand, evident decreases are observed for the other materials (Fig. S7). Besides, the XRD, SEM, TEM, and XPS results presented in Fig. S8–10 indicate that after the long-term test, the NiS₂/MoS₂ heterostructures still remain, though the content has reduced.

It is particularly noted that not only in an alkaline electrolyte, but also in 1 mol L^-1 PBS solution (as a neutral electrolyte), the NiS₂/MoS₂ heterostructures display excellent HER performances. As shown in Fig. 3a, the η₁₀ of the NiS₂/MoS₂ heterostructures is 157 mV, which is optimum when comparing with those reported for other transition metal sulfides (Table S2). Meanwhile, the η₁₀ of NiS₂, MoS₂, and nickel foam as electrodes are 224, 261, and 406 mV, respectively. As for the Tafel plots (Fig. 3b), the NiS₂/MoS₂ heterostructures exhibit a Tafel slope of 109 mV dec^-1, which is lower than those of NiS₂ and MoS₂, which suggests facile electron transfer for the HER. The superior HER activity and promoted charge transfer should be ascribed to the higher ECSA of the NiS₂/MoS₂ heterostructures, which is further confirmed by the Cdl investigations (Figs. 3c, 3d and S11). The improved charge transfer was also verified by EIS measurements (Fig. 3e), with the NiS₂/MoS₂ heterostructures displaying the smallest charge transfer resistance, compared to those of the other samples. In addition, the NiS₂/MoS₂ heterostructures also present excellent stability under neutral conditions, and no noticeable change in the polarization curve is observed after long-term electrolysis (Figs. 3f and S12).

According to the literature, Kuang and coworkers [14] performed DFT calculations to obtain the chemisorption free energies of hydroxide (ΔE_OH) and hydrogen (ΔE_H) on the nickel and sulfur sites on the (210) plane of NiS₂ and molybdenum, and on the sulfur and Mo-S edge sites on the (002) plane of MoS₂. It was concluded that the nickel and molybdenum sites at the heterointerface offered the best locations for OH and hydrogen adsorption. Furthermore, Zhang and coworkers [13] calculated the chemisorption free energies based on a more accurate model of MoS₂/Ni₃S₂ heterostructures, and deduced that the nickel site on the (101) plane of molybdenum-doped Ni₃S₂ and the molybdenum-sulfur edge site on the (002) plane of nickel-doped MoS₂ should be the sites where the HER proceeds. As shown in Fig. 4, based on the above findings, it is not difficult to conclude that the heterointerface of NiS₂/MoS₂, where the chemisorption free energies of the intermediate are obviously reduced, is the key factor for improving the catalytic activity; while the nickel sites act as centers where the dissociation of H₂O occurs, the molybdenum sites favor the absorption of hydrogen-containing groups.

In summary, based on previous work, we have prepared a non-noble-metal and efficient catalyst, namely, NiS₂/MoS₂ heterostructures, composed of highly dispersed NiS₂ nanoparticles and MoS₂ nanosheets that were in-situ grown on a nickel foam by using a one-step millisecond-laser-direct-writing method. This technique is not only simple, but also reduces the sizes of the two phases, so that the unit volume contains more heterointerfaces that are exposed. The combined advantages of a 3D porous structure, highly conductive self-supporting electrode, high specific surface area, and most importantly, synergistic effect at the heterointerface have led to greatly enhanced HER performances in both alkaline and neutral electrolytes, which suggest high promise for applications related to industrialized electrochemical H₂ production. Though the acidity properties require further improvement, this work provides a new pathway of engineering the interfaces of composites for developing low-cost and highly efficient electrocatalysts for water splitting.